Such a sad episode and Im sad they still haven't caught their killer. Those kids just did a nice deed and possibly gave this guy a ride. I think the show said that she was raped. If so, why don't they use the new DNA testing that can catch this guy especially since they think he has been in prison before???
At the time they did this show, it wasn't advanced enough but now it can be done. In fact alot of these cases could be solved.

It really depends on the cirumstances of the case. I've never worked in forensics before, but I know from what I've read and what I've seen on TV that, in some cases, they can go back and get DNA for testing. In many cases they cannot because the evidence has been misused and destroyed. They have exumed bodies for cases before, but I guess they cannot do that all the time because of the cost and resources involved. Maybe it depends on what the family of the victim wants. Many cases are also closed and have to be reopened before testing can be done. So there are many reasons why the authorities will or won't look into solving older cases involving DNA.